1. Field of the Invention
The present invention relates to an optical pickup for reading or recording information on a recording surface of a first optical record medium such as a compact disk or a second optical record medium such as a digital video disk, each of which has a protection film different in thickness from each other.
2. Description of the Related Art
There is known an optical system for recording and reproducing digital information including video data on an optical record medium. The optical system records and reproduces the digital information recorded onto the optical record disk in the higher density compared with conventional compact disk.
In order to reproduce the optical record disk, in which the digital video information is recorded in the high density, an optical pickup system, the resolution of which is so high as to reproduce the information recorded in the high density, is used.
There are two methods of increasing the resolution as follows:
(1) According to first method of increasing the resolution, the wave length of a light beam emitted by a light beam source is decreased. A semi-conductor laser, in which the wave length of a light beam is so short as to achieve the high resolution, has already been used.
(2) According to second method of increasing the resolution, the numerical aperture of an objective lens is increased. In case of increasing the resolution by increasing the numerical aperture, a detection signal obtained from the pickup system is influenced by the effect of aberration.
Particularly, if the recording surface of the optical record disk is not vertical to the optical axis of the objective lens due to the curve or the tilt of the optical record disk, the detection signal is considerably influenced by the effect of the aberration and it is difficult to increase the numerical number.
In order to overcome this difficulty, new disk standard is proposed. According to the new disk standard, an optical disk, which includes a protection film having thickness (0.6 mm as an example) less than that (1.2 mm) of the conventional optical disk, is proposed. Namely, since the effect of aberration due to the curve or the tilt of the optical record disk is decreased when the thickness of the optical disk is decreased, the effect of the aberration is decreased when the thickness of the optical disk is half of that of the conventional optical disk.
Therefore, when the thickness of the optical disk is half of that of the conventional optical disk, a small light spot is converged on the recording surface of the optical record disk by using an objective lens having an numerical number greater than that of the conventional disk and then information recorded on the recording surface of the optical disk in the high density can be reproduced by using the small light spot.
However, if a compatible player, which can reproduce the conventional optical disk and the optical disk based on the new optical disk standard, is constructed, the optical pickup of the compatible player should read information recorded on the conventional optical disk and the optical disk based on new optical disk standard.
However, if the objective lens of the optical pickup provided in the compatible player is arranged so that a light spot of a light beam converged by the objective lens on the optical disk having a protection film thickness less than that of the conventional optical disk is minimized, the light spot of the light beam converged on the conventional optical disk becomes broader and it is impossible to reproduce the information recorded in the conventional optical disk by using the light spot.
In order to overcome the problems, there is an optical pickup, which uses a bifocal lens, which consists of a hologram.
According to the optical pickup, which uses a bifocal lens, a laser beam emitted from the laser diode is reflected by a half-mirror and converted into a parallel light beam by a collimator lens. The parallel light beam is divided by an optical device, which consists of a hologram, into a non-diffracted light, which is a portion of the light beam other than diffracted lights and a first order diffracted light. The non-diffracted light is converged onto a recording surface of the conventional optical disk and the first order diffracted light is converged onto a recording surface of the optical disk based on the new optical disk standard by an objective lens, which is integrally formed with the optical device.
However, this optical pickup, which uses a bifocal lens, is disadvantageous in that it is complex in structure and is constructed on a large scale, since an objective lens is integrally formed with the heavy optical device and the strong driving force of an actuator for performing the focus control and the tracking control is needed.